A very wide variety of biochemical processes involve movement of protons and electrons, called proton-coupled electron transfer (PCET). This is particularly evident in the actions of heme enzymes, including cytochromes P-450, peroxidases, catalases, and many other enzymes. Both substrate oxidation and O2 or H2O2 activation by these enzymes require precise control of the movement of protons and electrons. The complexity of a full biological system, even in vitro, makes it difficult to analyze the features important to PCET. This proposal presents an experimental program that will develop a detailed mechanistic understanding of PCET in biomimetic systems relevant to the high-valent ferryl oxo intermediates compound I and compound II of these enzymes. Of particular importance are mechanisms where the transfer of protons and electrons occurs in a single kinetic step, as concerted proton-electron transfer (CPET). The CPET mechanism can avoid high-energy intermediates that would be otherwise be generated upon separate proton transfer (PT) or electron transfer (ET). An important distinction being made in this proposal is the concept of separated concerted proton-electron transfer (sCPET), in which the proton and electron to originate from or terminate at separate sites. This contrasts with hydrogen atom transfer (HAT) reactions where both PT and ET proceed from one single species to another single species. Herein, we propose electrochemical, spectroscopic, and chemical kinetic investigations using well-defined high-valent iron-heme (porphyrin)-oxo model complexes and select non-heme iron oxo complexes. Key thermodynamic information to be derived includes the acidity (pKa) and the reduction potential (E) of ferryl oxo and related intermediates using different porphyrin and axial ligands. A comprehensive kinetic profile for protonation and reduction of the ferryl oxo functional group will be developed using proton and electron donors that span a range of strengths, and Eyring analyses will provide the free energy barriers for these transformations. This will enable discrimination between sCPET and isolated ET-PT or PT-ET mechanisms in the biomimetic systems studied, and will provide an experimental foundation for extending the concept of sCPET to complex biological systems that incorporate ferryl heme and non-heme iron oxo intermediates.
Iron-containing enzymes execute remarkable chemical transformations related to cellular energy storage and conversion, photosynthesis, and neutralizing harmful materials. The work proposed here will provide greater understanding of how these enzymes achieve these tasks by developing simplified experimental models of these chemical transformations. Studies of these chemical model systems will provide important new insights into the functions and functioning of the full biological systems.